Operating Segment Information - Summary of Selected Information by Operating Segment (Details) - USD ($) $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2020	Sep. 30, 2019
Revenues
Net premiums earned	$ 216,630			$ 211,255			$ 640,946	$ 623,485
Net investment income	16,543			17,532			51,238	51,095
Net realized gains	1,512			3,211			14,555	17,043
Net unrealized gains (losses) on equity securities	28,126			4,906			(27,564)	47,214
Consolidated revenue	262,811			236,904			679,175	738,837
Net Earnings
Net underwriting income	1,152			13,731			42,554	51,096
Net investment income	16,543			17,532			51,238	51,095
Net realized gains	1,512			3,211			14,555	17,043
Net unrealized gains (losses) on equity securities	28,126			4,906			(27,564)	47,214
General corporate expense and interest on debt	(4,569)			(4,444)			(12,118)	(14,725)
Equity in earnings of unconsolidated investees	8,745			4,011			18,359	17,793
Earnings before income taxes	51,509			38,947			87,024	169,516
Income tax expense	9,122			6,623			13,738	31,252
Net earnings	42,387	$ 92,166	$ (61,267)	32,324	$ 40,467	$ 65,473	73,286	138,264
Casualty Segment
Revenues
Net premiums earned	143,002			140,423			421,637	415,667
Net Earnings
Net underwriting income	13,993			2,370			22,403	14,652
Property Segment
Revenues
Net premiums earned	45,380			41,476			135,115	120,194
Net Earnings
Net underwriting income	(19,951)			6,202			(3,876)	14,012
Surety Segment
Revenues
Net premiums earned	28,248			29,356			84,194	87,624
Net Earnings
Net underwriting income	$ 7,110			$ 5,159			$ 24,027	$ 22,432